Due to the environment-friendly efforts to reduce soot emissions of diesel engines, the need arises to determine the concentration of soot particles in the exhaust gas in a simple manner. In particular, monitoring the soot level downstream from a diesel particulate filter (DPF) during the operation of the vehicle is useful. In addition, it is necessary to predict the load of a diesel particulate filter for regeneration monitoring to achieve high system reliability.
To determine the soot concentration in the exhaust gas of internal combustion engines, a sensor having a device for detecting soot particles may be installed in the exhaust pipe.
German Patent documents nos. 101 33 384 A1 and 33 04 548 A1 discuss a resistive particle sensor, which has at least one non-conductive substrate element, measuring electrodes being situated on a substrate element. The measuring electrodes may be implemented in an interdigital comb structure. In an interdigital comb structure, each measuring electrode is formed by a series of individual finger electrodes, which are electrically connected to one another. The finger electrodes of both measuring electrodes alternatingly engage with one another, hence the designation “interdigital comb structure.” Deposition of particles on the measuring surface between the electrodes, known as leakage current surface, results in a change in conductivity or impedance of the measuring surface between the fingers of the electrodes. For example, the resistance, the real part of the impedance, decreases with increasing particle concentration on the measuring surface. Alternatively, an increasing current at constant voltage applied between the measuring electrodes may be measured. The deposition, i.e., the deposition rate of particles, may be derived from the change in the particular measured quantity—the sensor signal.
This measuring method corresponds to an accumulating measuring principle, and the sooted sensor surfaces must therefore be freed of the conductive soot particles from time to time whenever a defined saturation current or another threshold value is attained. A high voltage to burn the soot particles via the current flow may be applied between the electrodes for regenerating the sooted surface. Alternatively, an integrated heater may heat the sensor affected by soot, so that the accumulated soot is fully burned off. After the soot particles have been burned off, the sensor is in its original state again, and a new measuring cycle including re-deposition and measurement of particles is thus made possible. Measuring and regeneration phases thus always alternate over time.
One disadvantage of this procedure is that no new deposition of particles is possible during the burn-off. Even after regeneration, soot cannot accumulate immediately; due to its thermal inertia, the sensor needs a certain time for the exhaust gas to bring the sensor element to its working temperature. Since no soot may accumulate during regeneration and the subsequent cooling phase of the sensor, the sensor is insensitive to any soot concentration present during these phases. Therefore, a measuring phase that is as long as possible is desirable. At the same time, the measured value must be large enough to enable early and meaningful determination of the particle concentration.